摘要：SummaryMarimo (lake ball) is an uncommon ball-like aggregation of the green alga,Aegagropila linnaei.AlthoughA. linnaeiis distributed in fresh and brackish waters in the northern hemisphere, marimo colonies are found only in particular habitats. Here, we report the bacterial communities inside various sizes and aggregating structures of natural marimo collected from Lake Akan, Japan. We observed multi-layers composed of sediment particles only in the sizable radial-type marimo with >20 cm diameter and not in the tangled-type marimo. The deeper layers were enriched byNitrospira, potential sulfur-oxidizing bacteria, and sulfate-reducing bacteria. Microorganisms of the multi-layers would form biofilms incorporating nearby sediment, which would function as microbial “seals” within large radial-type marimo. These findings provide clues to deciphering the growth of endangered marimo.Graphical abstractDisplay OmittedHighlights•The radial type of marimo (lake ball) can grow to over 20 cm in diameter•The sizable radial-type marimo develops the internal multi-layers and hollow structure•The layers provide different diverse microbiomes and structural strength•The internal multi-layers support the massive growth of the radial-type marimoEcology; Microbiome; Algology; Omics
